1. Field of the Invention
This invention relates to a method of and an apparatus for image processing which are applicable to an image display, a copier, a facsimile and the like, and more particularly to a method of and an apparatus for image processing in which a color image signal (made up of image signal components representing image data on respective pixels) is converted into a monochromatic (black and white) image signal and a monochromatic image output is obtained on the basis of the monochromatic image signal.
2. Description of the Related Art
Recently as the color scanner becomes less expensive and/or by the advent of a digital camera and the like, it becomes feasible to easily make a color image signal by the use of various image input unit, and accordingly, a chance for the display, copier, facsimile or the like to output a monochromatic image on the basis of a color image signal has been increased.
When a monochromatic image is to be output on the basis of a color image signal, the color image signal has to be once converted to a monochromatic image signal by means of an image processing apparatus as disclosed, for instance, in Japanese Unexamined Patent Publication No. 6(1994)-59657.
In the method of conversion disclosed in Japanese Unexamined Patent Publication No. 6(1994)-59657, the color image signal is divided into a hue signal, a brightness signal and a chroma signal and a tone value for a pixel is determined taking into account the hue of the pixel in addition to the brightness of the pixel according to gray tables (tone conversion tables) which have been prepared hue by hue taking into account the fact that colors at the same brightness can be viewed different in brightness when hues are different.
For example, when a color image signal component L*in for a pixel is input as shown in FIG. 8A, a monochromatic image signal component L*Rout is output according to a gray table for red LUTR when the hue represented by the color image signal component L*in is red, and a monochromatic image signal component L*Gout is output according to a gray table for green LUTG when the hue represented by the color image signal component L*in is green. As a result, when two regions which are of the same brightness and of different hues exist side by side on a color image, the two regions are expressed different in brightness on a monochromatic image, whereby the difference in hue on a color image can be expressed on a monochromatic image, and information on brightness which can be visually detected on the color image can be preserved on the monochromatic image. That is, halftone image signal components of a color image signal can be clearly expressed by monochromatic image components.
However, the method of conversion disclosed in Japanese Unexamined Patent Publication No. 6(1994)-59657 is disadvantageous in that the monochromatic image signal component L*Gout according to a gray table for green LUTG for a green color image signal component L*Gin and the monochromatic image signal component L*Rout according to a gray table for red LUTR for a red color image signal component L*Rin representing brightness different from that of the green color image signal component L*Gin can become equal to each other as shown in FIG. 8B. That is, pixels which are different from each other in both hue and brightness on a color image can be expressed in the same tone on a monochromatic image, whereby information on a difference in hue between two adjacent regions is lost on a monochromatic image.